This invention relates to coating compositions and more specifically to corrosion-resisting coatings which can be applied directly to various surfaces particularly metal either as a high or low gloss, self-priming corrosion-resistant topcoat.
Metal surfaces require the protection of coatings especially when the surfaces are exposed to a corrosive environment. Metal surfaces of aircraft, for example, are exposed to seawater which require protection from corrosion. Specifically, aircraft, e.g., Navy aircraft, are exposed to seawater spray in addition to various acid-forming gases such as sulfur dioxide, carbon dioxide, etc. Moreover, in addition to aircraft, various machinery and equipment in the industrial environments, where fossil fuels are used, need protection against corrosion. It is important therefore that the coating be resistant not only to corrosion but also various chemicals, the weather and at the same time be flexible and have good adhesion to the metal substrates.
The coating systems in the prior art comprise one or more films, i.e., an undercoat, a sealant and a topcoat. Aircraft, for example, traditionally have been coated with high performance two-component protective films consisting of a primer and a polyurethane topcoat. The type of primers used on the aircraft are designed to adhere to the metal surface and help to improve the adhesion of the topcoat and together prevent corrosion of the metal. However, these undercoats require a topcoat, since the undercoats lack flexibility especially at low temperatures (-60.degree. F.) resulting in extensive cracking particularly in high flexed areas of the aircraft. In addition, the undercoats usually lack weather resistance and generally cannot be formulated in different colors required for aircraft.
The alkyd coatings of this invention, however, provide the necessary corrosion resistance, the required degree of flexibility, the desired optical properties, and the needed resistance to weather and various chemicals. Presently, to obtain any or all of these characteristics the prior art multi-film coatings generally required a dry-film thickness ranging up to about 0.005 inches, e.g., up to about 10 mils or more which adds considerable weight to the aircraft. In addition, these prior art coatings are time consuming to apply particularly since there is need for a drying period between each application. Further, the removal of a two-coat system can be difficult and time consuming which also generates high levels of volatile organic compound (VOC) emissions during the operations.
In accordance with this invention, the corrosion-resistant coatings comprise an alkyd resin binder in combination with a unique mixture of corrosion-inhibiting pigments consisting essentially of (1) an alkaline earth metal phosphate, particularly the zinc-bariumphosphates, (2) zinc salts of benzoic acid or substituted benzoic acid, and (3) an aluminum triphosphate. All three of these compounds are essential, in the stated relative proportions, to provide a single coat with the necessary corrosion resistance and adhesion characteristics required of a good top coat. In addition, titanium dioxide (TiO.sub.2) including spherical TiO.sub.2 particles, e.g., vesiculated beads of TiO.sub.2 is included as a pigment together with these metal compounds. The coating compositions of this invention may be applied, as one coat, directly onto various hard surfaces such as metal and do not require an undercoat to provide a corrosion-resistant finish with the desired optical and adhesion properties.